Electric heating – Metal heating – By arc
Reexamination Certificate
2000-08-25
2003-05-06
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06559416
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to wire welding, and more particularly to apparatus that conducts welding power to a weld wire.
2. Description of the Prior Art
To properly perform wire welding, it is imperative that the conduction of welding power from a welding gun to the weld wire be controlled. Historically, welding power conduction occurred inside a copper contact tip in the gun. The contact tip had a hole through which the weld wire passed. The relatively long length of the contact tip and a small clearance between the contact tip hole and the weld wire increased the probability of smooth and consistent power conduction.
However, it has long been recognized that a problem existed in conducting the welding power from the contact tip to the weld wire. The problem had several aspects. The first was the lack of constant contact between the contact tip and the weld wire. Under the great majority of operating conditions, the weld wire was indeed in contact with the contact tip. Nevertheless, in some instances there were momentary conditions in which no contact occurred. In those situations, the welding current could arc across the clearance between the contact tip and the weld wire. The arcing was detrimental to the welding process.
Some prior welding machines relied on the inherent cast of the weld wire to produce a constant contact between the contact tip and the weld wire. However, non-uniformities of the weld wire cast, combined with movements in space of the welding gun during operation, inevitably resulted in momentary breakages of contact between the contact tip and the weld wire.
A second aspect of the welding power conduction problem concerned the size of the area of the interface between the contact tip and the weld wire. As mentioned, for the great majority of time during a welding process, the weld wire was in contact with the contact tip. However, even during those times of contact the areas of contact were usually very small. The typically large welding currents resulted in very high current densities, which tended to produce tiny microwelds between the contact tip and the weld wire. The small welded areas between the contact tip and weld wire were usually quickly remelted or mechanically broken. However, a stick-slip type of weld wire feeding resulted. The stick-slip feeding of the weld wire caused by repeated microweld creation and breakage contributed to poor welding performance.
A third aspect of the problem involving welding power conduction in prior welding guns was that is was possible, and even likely, that at some times during a welding operation the weld wire contacted the contact tip at more than one interface. At any particular moment, the locations of the interfaces were randomly variable along the length of the contact tip. Because of the greater electrical resistance of the weld wire than the contact tip, the amount of welding power conduction was greatest at the interface that was closest to the downstream end of the contact tip. As a result, almost all of the preheating of the weld wire occurred between the downstream interface and the workpiece. Since the interface furthest downstream invariably changed location within the contact tip during a welding operation, the amount of preheating of the weld wire also varied. Changes in the weld wire preheating changed the characteristics of the welding arc and therefore were potentially detrimental to satisfactory welding.
Further aspects of the problems involving prior contact tips included wear of the tip clearance hole. The wear contributed to random and inconsistent interfaces between the contact tip and the weld wire. The weld wire frequently had shavings and other debris clinging to it. When the foreign matter entered the contact tip it could become trapped and prevent the weld wire from feeding properly.
Related problems included spatter from the welding arc that could fly up and stick to the end of the contact tip, where it was likely to build up and stick to the weld wire. Burnback caused by a failure anywhere in the welding machine could destroy any contact tip. Similarly, excessive heat from the welding arc could cause a contact tip to reach a plastic state and wear out prematurely.
It is therefore well recognized that a contact tip can fail in a number of ways. For example, U.S. Pat. No. 3,112,393 mentions the tendency of a weld wire to arc and fuse to the contact tip, which results in a short life for the contact tip.
Prior contact tips were often treated as low cost consumable items. Yet, a contact tip failure could disrupt an entire production line. In addition, a malfunctioning contact tip caused inconsistent or improper operation of other components of a welding machine such as the weld wire feeder or the gas flow. Frequently, other welding machine components than the contact tip were searched and mis-diagnosed as the source of the problem.
Accordingly, numerous attempts have been made to solve one or more aspects of the contact tip and the weld wire interface problem. For example, to assure a constant interface, power carrying wire feed rollers were used in some welding machine guns to ensure a constant location of the interface. The feed rollers were not the complete answer, however, because additional interfaces were present at the contact tip. As mentioned, the electrical resistance of the contact tip was less than the resistance of the weld wire. Hence, even with conductive feed rollers the most downstream interface between the contact tip and the weld wire remained random and uncontrollable.
Other proposed solutions included a non-circular clearance hole in the contact tip, such as is shown in U.S. Pat. No. 5,635,091. U.S. Pat. No. 5,278,392 shows a contact tip that is covered with a graphite cover. U.S. Pat. Nos. 4,945,208 and 4,978,831 teach a chrome plated bushing covering the downstream end of a contact tip to eliminate carbon residue on the tip.
A solution for arcing and microwelding proposed by U.S. Pat. No. 3,112,393 is a contact tip having a helical clearance hole for the weld wire. A somewhat similar proposal is described in Great Britain patent 2,074,069A. That patent shows a contact tip with one or two projections inside the contact tip clearance hole. The projections force the weld wire to change direction inside the contact tip and thereby assure good contact with the contact tip. Another helix-related contact tip is described in U.S. Pat. No. 4,733,052, in which a contact tip is in the form of a rectangular strip wound into a helix. A central longitudinal opening through the helix is smaller than the diameter of the weld wire, such that the weld wire is resiliently squeezed by the contact tip.
U.S. Pat. No. 4,563,569 describes a welding gun in which a weld wire guide tube is transversely pivotable within the gun body. Springs within the gun urge the guide tube transversely such that the weld wire is urged into transverse contact with the clearance hole in the contact tube, which is fixed relative to the gun body.
In U.S. Pat. No. 3,102,947, arcing between the contact tip and the weld wire is prevented by lining the contact tip with an insulative sleeve. Welding power is transferred to the weld wire upstream of the contact tip.
U.S. Pat. No. 4,988,846 describes a contact tip with an arcuate groove cut in the wall between two coaxial clearance holes for the weld wire. The weld wire contacts and is deflected by the groove surface, thus assuring an intimate interface between the contact tip and the weld wire. The same principal is used in U.S. Pat. No. 3,142,746, which discloses a ball that partially closes the clearance hole in the contact tip. The ball is biased radially against the weld wire to assure a constant interface between the contact tip and the weld wire.
U.S. Pat. No. 4,731,518 teaches a welding gun with a reverse radius in the head tube between the gun handle and the contact tip. The reverse radius causes the weld wire to be biased against the clearance hole in the contact tip.
Japan patent JP-11170052-A describe
Beeson Richard W.
Holverson Todd E.
Hutchison Richard M.
Steenis Robert D.
Uecker James L.
Croll Mark W.
Illinois Tool Works
Pilarski John H.
Shaw Clifford C.
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